In recent years, various kinds of information apart from speech, such as images and data, have come to be transmitted in radio communications, and particularly in mobile communications. With the demand for still higher-speed transmission expected to continue to grow in the future, there is a need for a radio transmission technology that achieves high transmission efficiency through more efficient use of limited frequency resources in order to perform high-speed transmission.
One radio transmission technology capable of meeting such a need is OFDM (Orthogonal Frequency Division Multiplexing). OFDM is a multicarrier transmission technology that performs parallel transmission of data using a plurality of subcarriers, and is known for such features as high frequency efficiency and reduced inter-symbol interference in a multipath environment, and for its effectiveness in improving transmission efficiency.
Studies have been carried out into performing frequency scheduling transmission and frequency diversity transmission when this OFDM is used in a downlink, and data for transmission to a plurality of radio communication mobile station apparatuses (hereinafter referred to simply as mobile stations) is frequency-domain-multiplexed on a plurality of subcarriers.
In frequency scheduling transmission, a radio communication base station apparatus (hereinafter referred to simply as a base station) allocates subcarriers adaptively to mobile stations based on the received quality of each frequency band at each mobile station, enabling a maximum multi-user diversity effect to be obtained, and extremely efficient communication to be performed. Such frequency scheduling transmission is mainly suitable for data communication when a mobile station is moving at low speed, or for high-speed data communication. On the other hand, frequency scheduling transmission requires feedback of received quality information from each mobile station, and is therefore not suitable for data communication when a mobile station is moving at high speed. Frequency scheduling transmission is normally performed in transmission time units called subframes for individual Resource Blocks (RBs) in which a number of adjacent subcarriers are collected together into a block. A channel for performing this kind of frequency scheduling transmission is called a Localized Channel (hereinafter referred to as Lch).
In contrast, in frequency diversity transmission, data for each mobile station is allocated distributed among subcarriers of an entire band, enabling a high frequency diversity effect to be obtained. Also, frequency diversity transmission does not require received quality information from a mobile station, and is thus an effective method in circumstances in which use of frequency scheduling transmission is difficult, as described above. On the other hand, frequency diversity transmission is performed without regard to received quality at mobile stations, and therefore does not provide the kind of multi-user diversity effect obtained with frequency scheduling transmission. A channel for performing this kind of frequency diversity transmission is called a Distributed Channel (hereinafter referred to as Dch).
It is possible that frequency scheduling transmission in an Lch and frequency diversity transmission in a Dch may be performed simultaneously. That is to say, an RB used for an Lch and an RB used for a Dch may be frequency-domain-multiplexed on a plurality of subcarriers of one OFDM symbol. At this time, mapping between each RB and Lch, and mapping between each RB and Dch, are set in advance, and which RB is used as an Lch or a Dch is controlled in subframe units.
Another idea that has been studied is to further divide an RB used for a Dch into a plurality of subblocks, and form one Dch by means of a combination of different RB subblocks. At this time, a plurality of Dch's with consecutive channel numbers are mapped to a plurality of RBs that are consecutive in the frequency domain (see Non-Patent Document 1, for example). Non-patent Document 1: R1-072431 “Comparison between RB-level and Sub-carrier-level Distributed Transmission for Shared Data Channel in E-UTRA Downlink”3GPP TSG RAN WG1 LTE Meeting, Kobe, Japan, 7-11 May, 2007